1. Field of Art
This invention relates to diagnostics of spark ignition engines, and more particularly to versatile, dual-threshold signal conditioning of a vehicle low coil signal.
2. Description of the Prior Art
As is known, the traditional breaker-point ignition system charges the primary of the high voltage coil by having the points thereof closed during the dwell period, after which the points open causing an inductive kick in the primary which in turn results in a high, ignition spark inducing voltage in the secondary of the coil for application through the distributor to the various spark plugs. The primary voltage (also called the low coil signal) thereafter rings sinusoidally until it damps out, and becomes steady at substantially the vehicle battery voltage before the points again close initiating the next dwell period. In more modern, electronically controlled, high voltage ignition systems, the electronic control over the coil primary begins at a low, near-ground potential but then rises to a potential on the order of half the battery voltage before electronic current limiting causes this voltage to remain fairly steady until the end of the dwell period; then the circuit is broken so that the inductive kick will occur in the primary to create the spark-inducing voltage of the secondary of the coil. This is followed by ringing, in the same fashion as in breaker-point ignition systems.
In prior art vehicle diagnostics, it has been known to provide an accurate measure of the dwell time (etc.) by threshold detecting the rise and fall of the low coil (or coil primary) voltage. In the past, the characteristics of the breaker-point ignition system rendered this relatively simple since a single voltage threshold (on the order of 3 or 4 volts) could be used to sense the end of the dwell period when the voltage exceeded that threshold, or the beginning of the dwell period when the voltage was reduced below that threshold. In the various modern systems, however, the voltage is initially at ground at the start of the dwell period, and may raise to some voltage varying between 3 volts and 8 volts (in normal 12 volt ignition systems) before the primary is broken to develop the inductive kick. This voltage range compares nearly identically with battery voltages which can obtain during cranking of the engine with a weak battery, which may be on the order of only 8 or 9 volts. Thus the threshold detecting is hampered not only by variations in the voltage level near the end of the dwell period for different types of high voltage, electronically-controlled systems, but also because of its similarity to the battery voltage which the coil primary assumes after its oscillatory ringing period. Also, cranking with a weak battery compared with high speed operation with a good alternator can cause the battery voltage variations of various engines to be too divergent for fixed threshold comparison.